7/04/2010

One thing that I have found most annoying about the movie Avatar is the chorus of praise for its "scientific accuracy." I don't know of any movie short of Star Trek that has tried so hard to convince its fans that it's based on real science, and fallen so short. So I thought it would be useful to just make an archive showing that the movie is not science-fiction; it's science mixed with fantasy. This week, we'll look at the big picture by focusing on the planet-moon Pandora.

If you look at the technical details behind the scenes, Avatar did do an impressive job of trying to create a realistic planet. Recent computer modeling shows that the Alpha Centauri system is now considered a good candidate for Earth-like planets. Even a planet-sized moon orbiting a gas giant is also a possibility that many astronomers consider plausible. However, it's not without problems.

In the movie, Pandora is in orbit around a gas giant called Polyphemus. While we don't know for sure if there are planets in the Alpha Centauri system, we know that a gas giant would be almost impossible. Gas giants are formed by small particles of ice and dust clumping together over time. Eventually, the particles form a small core that attracts more and more dust and then gases until it forms a planet. Tidal forces of two suns would prevent this kind of planet formation in a binary system, because they would disperse the initial clumping. In addition, direct observation of Alpha Centauri would have detected any large gas giants by now. In twenty years, with new technology and techniques adding to our ability to observe Alpha Centauri, Avatar's portrayal of a huge life-bearing moon orbiting a gas giant in Alpha Centauri will seem about as realistic as the beautiful Venusian women in Abbott and Costello Go to Mars. But let's assume for a moment that the gas giant did exist in Alpha Centauri, and there was a moon the size of a planet orbiting it. Even then, we have problems.

One problem with a planet-sized moon is that moons orbiting large planets are tidally locked. That means that the moon takes as long to rotate on its axis as it does to orbit its planet. One side of the moon always faces the planet it orbits around, while the other faces out into space. This doesn't make life impossible, but it does make it difficult. The question of whether Pandora is tidally locked is actually something I spent some time on and had trouble verifying. One article on Pandora claimed that the planet is tidally locked, which explained the bioluminescense evolving due to long nights. However, another source at avatar-forums.com said that all official information, including the original script and treatment, indicate that Pandora is in fact not tidally locked. Either way, we have problems. A tidally locked planet can lead to days so long that the planet would be uninhabitable - burning hot on one side and freezing cold on the other, while intense winds from the temperature differential sweep the surface like a never-ending hurricane. Not having a tidally locked moon on a planet as large as Polyphemus is monstrously unrealistic.

In addition, the tidal forces would be tremendous. For example, the surface of Jupiter's moon Io literally ripples up and down like waves on our oceans. The gravitational pull would also cause the surface of the moon to be constantly in flux, leading to volcanoes, earthquakes, and other seismic activity. In the movie, we see that Polyphemus has multiple moons, which would also increase the tidal effect.

Polyphemus' gravity would also turn Pandora into a virtual moving target, bombarded with objects drawn in by the gas giant's enormous gravitational pull. Asteroids and comets would all rain down on Pandora, causing explosions or dust clouds that would destroy any life that might exist.

Now obviously the astronomers that Cameron's team consulted were aware of all of these things and factored them in. If you read the official guide, it describes the volcanic activity, tectonic activity, meteor strikes, and intense magnetic activity that Pandora endures. The problem is that all of it is on paper. In the actual movie, we see what appear to be normal night and day cycles, and no indication of huge tidal eruptions or massive amounts of volcanic activity. The only evidence of the magnetic fields are the floating mountains and pretty Aurora Borealis effect. There's no way you would get the idyllic jungle paradise portrayed in the film. It's obvious that, once again, Cameron looked at reality, compared it to his fantasy, and threw science out the window to get his fantasy. Which is flawed science.

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comments:

Well, I guess its about as realistic as any popular science-fiction out there. It's weird to think how delicately balanced the planetary systems are. I was writing a science-fiction piece a while back and shudder to think of how impossible it all was. Badastronomy.com would have had a field day.

That's my point - "Avatar" isn't science fiction. It's fantasy mixed with some science. "Avatar" started it by proclaiming how accurate and well-researched it was. As for my making a better movie, I don't have to. Others have. Check out our list of the top ten sci-fi movies that didn't win Best Picture for a start.

In the last year a few scientific papers have been published suggesting that Neptune size gas giants can form, in situ, close to stars, easily within their habitable zones.

And only a few months ago, one scientific paper, written by Xie et al, even suggests that large gas giants, of Saturn, Jupiter or larger size, are very likely to form in close orbits around binary star systems like Alpha Centauri, through a process plantesimal piling. see http://arxiv.org/PS_cache/arxiv/pdf/1104/1104.1475v1.pdf

Of course there aren't any gas giants around Alpha Centauri, A or B because they would have been detected by now through radial velocity measurements, probably.

Io exhibits volcanic activity and the effects that your post discusses because it's involved in what's called an orbital resonance with Ganymede and Europa. Not all moons around gas giants are subject to this.

Furthermore, tidal lock is not going to be problematic in this case, as the object is in tidal lock to a planet, rather than a star. As the moon orbits the planet, so too will the sun move across its sky as it turns about its parent world.

Yes, there are faults with the film, but it's very hard not to make mistakes when you're creating something like this, and I think it's more a case of looking at a glass half full or half empty. :-)

Good overall point, but I think a few of the supporting details don't work. Tidal effects on a moon depend on how close it is to the planet, so you can put it wherever. There's no rule that says the moons have to be tidally locked. In fact, during formation the rule is that they have their own angular speed based on random fluctuations. A regular spin cycle would take care of temp fluctuations. But since Pandora has an ocean, the drag effects would eventually lock it. That could take centuries of course, and the farther out it is, the longer it takes. But I grant that it would be a huge coincidence for the planet to have Earth like night and day lengths, which seems to what is portrayed. And with a huge planet to block the sun, the moon would be in darkness half the time by the eclipse effect. How long would depend on its orbit time which if you take Jupiter as a model, that could a matter of days or weeks.

To be honest, this was the hardest article to write, because there seems to be a lot of debate among astronomers as to what kinds of planets could exist and where. I included links to science articles to support my arguments, but am willing to concede your points about tidal effects. But another problem was that there wasn't a clear consensus on whether Pandora is tidally locked. Some official sources say no, others yes.

"A tidally locked planet can lead to days so long that the planet would be uninhabitable - burning hot on one side and freezing cold on the other,"

Ummm.... No.

The day of a tidally locked planet is the same as its orbital period. Thebe has a .676 day period, Io 1.771 days, Mimas .94 days, Enceladus 1.371 days. So it is quite possible for a gas giant moon to have something in the neighborhood of a 24 hour day.

"In addition, the tidal forces would be tremendous."

It can be. As you mention Io has some extreme volcanism. But not all our gas giant moons have Io's volcanism. Another non-issue.

"Asteroids and comets would all rain down on Pandora, causing explosions or dust clouds that would destroy any life that might exist."

Which is why the surface of Titan is covered with impact craters. Except that it isn't. Like Earth, it has weather that erodes evidence of impacts. Also like the earth, it has a protective atmosphere.